[Python-Dev] TypeError messages

[Rob Cliffe]

On 19/02/2015 23:57, MRAB wrote:
> On 2015-02-19 22:50, Serhiy Storchaka wrote:> Different patterns for
> TypeError messages are used in the stdlib:
> >
> >       expected X, Y found
> >       expected X, found Y
> >       expected X, but Y found
> >       expected X instance, Y found
> >       X expected, not Y
> >       expect X, not Y
> >       need X, Y found
> >       X is required, not Y
> >       Z must be X, not Y
> >       Z should be X, not Y
> >
> > and more.
> >
> > What the pattern is most preferable?
> >
> Stylistically, if the first part is in the active voice, then the
> second part should also be in the active voice:
>
>     expected X, but found Y
>
> The active voice tends to be simpler and easier to parse than the
> passive voice.
>
> I think that the word "but" adds clarity here.
>
> Strictly speaking, that message is OK only if it's expecting X itself;
> if, in fact, it's expecting an instance of X, then you should really be
> saying something along the lines of:
>
>     expected X instance, but found Y instance
>
> or:
>
>     expected instance of X, but found instance of Y
To me that is a purist argument, but in practice
         expected int, but found tuple
is perfectly clear (I have received such messages myself!), and extra 
verbiage is just clutter.
Identifying Z may be helpful however, so my feeling would be to stick to 
2 patterns - the one above and
     Z should be int, not tuple
(You may prefer e.g. "got" to "found", or "must" to "should".  But 
ideally the usage should be the same throughout.)
Rob Cliffe
>
>
> > Some messages use the article before X or Y. Should the article be
> > used or omitted?
> >
> Messages tend not to be complete sentences anyway, so I think that it
> would be fitting to omit articles.
>
> > Some messages (only in C) truncate actual type name (%.50s, %.80s,
> > %.200s, %.500s). Should type name be truncated at all and for how
> > limit? Type names newer truncated in TypeError messages raised in
> > Python code.
> >
> Truncating type names is probably not a good idea.
>
> > Some messages enclose actual type name with single quotes ('%s',
> > '%.200s'). Should type name be quoted? It is uncommon if type name
> > contains spaces.
> >
> I think that it should be quoted only if it's expecting those
> characters, e.g. if it's expecting a closing parenthesis, then it
> should say ')'. If, on the other hand, it's expecting a certain type,
> then it should give that type unquoted.
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